1. Field of the Invention
The present invention relates to a polyester film with a thermosetting resin adhesive layer, which is adapted to form a protective coat on a metal surface of a can, for example, a surface of a metal sheet for forming a can body, such as a surface-treated steel sheet both end edges of which are overlapped to each other to weld them, thereby forming the can body, or an outer side wall of a closed-end cylindrical can having an opening at one end thereof and formed by drawing with ironing or the like from a metal sheet for forming cans such as an aluminum alloy sheet, and a production process thereof.
2. Description of the Background Art
As metal cans used as containers for beverage, there have been known welded cans obtained by welding both end edges of a strip-like can body blank obtained by cutting a surface-treated steel sheet for cans, which has a tin deposit, to each other to form a can body and attaching can ends separately produced to both open ends of the can body by double-seaming. In order to prevent the corrosion of the surface-treated steel sheet for cans as a base metal due to the contact of the base metal with the contents in a can and the change in the flavor of the contents, which is caused by the dissolving of metal components of the can, the welded cans as described above have heretofore been coated with a coating such as an epoxy-phenolic resin on the inside thereof. In addition, coating has been conducted on the outside of the can to impart a good aesthetic appearance.
Such welded cans have been produced by conducting baking in a heating oven whenever coating was completed on each side of the surface-treated steel sheet for cans. Therefore, in this production process, heat in the heating oven and vaporized organic solvents have a tendency to impair a working environment. In particular, the tendency becomes more pronounced on the outer surface of the can, which is subjected to multicolor printing, because baking is carried out in the heating oven whenever printing is conducted with each of two to four coatings of different colors.
It has thus been studied to cover both sides of the surface-treated steel sheet for cans with a polyester film to form protective coats. Such covering can prevent the surface-treated steel sheet from corrosion on the inside of the can, and moreover protect the flavor of the contents in the can from being changed due to components slightly dissolved out of the coating film on the can on the inside of which an internal protective coating has been applied and baked as described above. On the other hand, a good aesthetic appearance and/or desired indication can be given to the outer surface of the can without conducting coating by covering it with a printed polyester film. Therefore, such covering makes it unnecessary to conduct the coating and baking in the conventional production process and hence can be expected to the improve working environment.
In order to cover both sides of the surface-treated steel sheet for cans with polyester films, it is considered to press-bond the polyester films to the surfaces of the heated steel sheet.
However, the mere press bonding of the polyester films to the surfaces of the surface-treated steel sheet for cans heretofore in use under heat for a short period of time cannot achieve sufficient adhesion. Besides, the bonding of the polyester films to the surfaces of the surface-treated steel sheet through a usual thermosetting resin adhesive requires a long time for heating at a high temperature to obtain sufficient adhesion. However, if the surface-treated steel sheet has a tin deposit, such a heating treatment is accompanied by a disadvantage that tin in the tin deposit forms a tin-iron alloy together with a base steel sheet during the heating, so that the amount of pure tin is decreased, and the weldability of the surface-treated steel sheet is hence reduced.
There have also been known cans obtained by forming a closed-end cylindrical can having an opening at one end thereof by drawing with ironing or the like from a metal sheet for forming cans such as an aluminum alloy sheet and attaching a can end separately produced to the opening of the can by double-seaming. As with the case of the welded cans, such cans are also accompanied by a disadvantage that heat in the heating oven and vaporized organic solvents impair a working environment if coating by printing is conducted on the outside of the can to impart a good aesthetic appearance.
In the case of the closed-end cylindrical can, coating is first conducted on the inner side of the closed-end cylindrical can formed by drawing with ironing or the like from the metal sheet for forming cans, and baking is then effected. Thereafter, 2 to 4 coatings of different colors are then coated on the outer side of the can one by one, and baking is conducted with every coating. Therefore, it is considered to press-bond the polyester film under heat to at least the outer surface of the closed-end cylindrical can.
As with the case of the surface-treated steel sheet for forming can bodies of welded cans, however, the mere press bonding of the polyester films to the surfaces of the metal sheet for forming cans heretofore in use under heat for a short period of time cannot achieve sufficient adhesion. Besides, the bonding of the polyester film to the surface of the metal sheet through a usual thermosetting resin adhesive involves a disadvantage that it requires a long time for heating at a high temperature to obtain sufficient adhesion, resulting in unavoidable reduction in operating efficiency and increase in cost.